JavaScript Scope Challenge

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What is scope? Your explanation should include the idea of global vs. local scope.
  
  Scope is the area in which a variable is accessible. Variables declared at 
  the top level of an application have global scope and are accessible by all 
  code. While variables declared inside a function have local scope and are only 
  accessible by the code in that function. 

Why are global variables avoided?
  
  A global variable is essentially a variable that is shared among all the code 
  in the application. Which means function A can change the value of the 
  variable without function B knowing about it. Then when function B goes to use 
  the variable it will be in the wrong state and errors will occur. It's a bit 
  like when your roommate drinks all the milk and you are stuck eating dry 
  cereal.

Explain JavaScript's strict mode
  
  Strict mode instructs the JavaScript interpreter to be more harsh when 
  evaluating code. Some types of code which are technically valid but are 
  considered bad practice or likely to be the source of subtle errors are 
  considered errors by strict mode. Strict mode is your friend.

What are side effects, and what is a pure function?
  
  A pure function is a function that does not have any side effects. Side 
  effects are changes to state outside of the function's own local scope. A pure 
  function takes it's input via its arguments, does all its work in its own 
  local scope, and then provides its output via the return keyword.